Inhibitory effects of okadaic acid on thyrotropin and prolactin secretion from rat anterior pituitaries

Inhibition of protein phosphatases by okadaic acid and calyculin-A differentially modulates hormonal- and forskolin-stimulated formation of cyclic AMP in AtT-20 corticotrophs: effect of pituitary adenylate activating polypeptide and corticotropin-releasing factor

The effect of phosphatase inhibitors okadaic acid and calyculin-A on cAMP formation and adrenocorticotropic hormone (ACTH) secretion in AtT-20 corticotrophs was investigated. Both okadaic acid and calyculin-A inhibited dose-dependently the accumulation of cAMP in cells stimulated with pituitary adenylate cyclase activating factor (PACAP) and corticotropin-relating hormone (CRF). While in the case of okadaic acid the half-maximum inhibiting concentration was similar for both peptides (IC50 = 4 x 10(-7) M), it appeared that calyculin-A was about one order of magnitude more efficient in inhibiting the effect of PACAP than that of CRF (IC50 = 3.8 x 10(-9) M vs 2.0 x 10(-8) M, respectively). Importantly, the inhibitors blocked the activation by cholera toxin (which acts on Gs-like proteins) of cAMP formation, but failed to alter the effect of forskolin (which bypasses the receptor-G protein complex and activates adenylyl cyclase directly). Treatment of cells with calyculin-A significantly dampened adenylyl cyclase activity in cell membrane fraction, though to a lesser extent than it blocked cAMP formation in the whole cell. Both okadaic acid and calyculin-A inhibited CRF- and PACAP-induced secretion of ACTH. Our data hint that in AtT-20 corticotrophs, inhibition of phosphatases by modulating the state of phosphorylation of the receptor-G proteins complexes for CRF and PACAP, regulates cAMP formation and ACTH secretion.

Protein phosphatase 1 and 2A in tick salivary glands as assessed by responses to okadaic acid

Crude protein phosphatase activity was inhibited 80% by nanomolar okadaic acid (OA) in salivary glands of unfed ticks but only 40% in salivary glands of feeding ticks. An additional 40% of protein phosphatase was inhibited by micromolar OA in the salivary glands of feeding ticks but only 10% in salivary glands of unfed ticks. Cyclic AMP and OA alone or together increased the phosphorylation of multiple proteins in a plasma membrane-enriched 900 g supernatant fraction of tick salivary glands. Exogenous cyclic AMP stimulated increased incorporation of phosphate into proteins with approximate molecular weights of 109, 70, 64, 51, 48, 42 and 18.5 kDa. Micromolar OA in the absence of exogenous cyclic AMP stimulated increased incorporation of phosphate into proteins with apparent molecular weights of 109, 93, 74.5, 70, 51, 48, 42 and 18.5 kDa. Cyclic AMP and OA (10(-6) and 10(-9) M) stimulated significantly greater phosphorylation of an 18.5 kDa mol. wt protein above that observed in response to stimulation by OA (10(-6) and 10(-9) M) or exogenous cyclic AMP alone. Micromolar okadaic acid inhibited the amount and number of proteins but not volume of saliva secreted by whole ticks in response to stimulation by DA and theophylline. However, micromolar and nanomolar okadaic acid inhibited the ability of dopamine to stimulate fluid secretion by isolated salivary glands. Overall, the data support the existence of type 1 and 2A protein phosphatases in tick salivary glands and a role for protein phosphatases in modulating tick salivary secretion.